I've shot just one moose with 12 ga slug- I think it was a 2 3/4 inch Rem. Foster, but I'm not sure. I was working for F&G that summer, away from home, up on the Yukon. Nearby was a half-mile wide, mile long oxbow, in which I'd pre-season scouted 2 big bulls and a yearling, along with several cows with calves.
My weapon of necessity was a Win 1200, with single front bead, and I didn't think those 7 1/2s were going to work all that well on moose. The Ruby store had two 5 packs of slugs, which I bought, and used one to sight-check the gun on a 4 inch diameter washed out spruce with root wad, which I set upright on the sandbar. Nailed it with every shot from 25 to 150 yards, but with about 4' of height differential. I was ready to hunt!
Opening morning i killed the spike (he was running) with a broadside shot at about 35 yards. Wing shooting! The slug (a bit high!) perfectly centered the big artery just under the spine, leaving artery wall on both sides of the hole. Full penetration, no expansion. He made it about 100 yards before falling over, all bled out.
It was over a week before I could get him (butchered and packaged on site), down to Galena, in perfect outdoor aging weather. Wien Air (now defunct), had a hunting season back-haul special to Anchorage for $25. Frozen overnight in their big walk-in cooler in Galena. They even transshipped it with no extra charge to Kenai, where my wife picked it up and put in the small freezer i had told her to buy a week before season opened.
That might be the best moose I've ever eaten, and not just meat-wise.
The point being a 12 ga slug is perfectly adequate for moose within it's range capability.
There is more to this story, like how we rolled a grizzly swimming the Yukon with the boat ( Not F&G boat, not F&G time, any of the above) wake on the way to Galena, but that's already been posted a couple times. He wasn't a happy bear......
